The etching process of a printed circuit board (PCB) is as follows: applying an etchant on a pre-developed copper-clad laminate and etching away the unprotected, non-conductor part of the PCB, in order to form a circuit. The etching of the non-conductor part utilises redox reactions between the etchant and the copper. Said pre-developed copper-clad laminate is made in previous processes and has a pattern.
At present, acidic cupric chloride etchant and alkaline cupric chloride etchant are two widely applied etching systems in industry. The acidic cupric chloride etchant employs cupric chloride (CuCl2) as the copper etching agent, and applys an acidic oxidation system to achieve the regeneration of the copper etching agent. The copper etching agent of the alkaline cupric chloride etchant is copper(II) ammonia complex Cu(NH3)4Cl2 formed from the complexation reaction between cupric chloride and ammonium hydroxide, and the etching agent is regenerated via a reaction involving oxygen, NH4+ and Cl−.
The Etching Mechanism of Alkaline Cupric Chloride Etchant
The main components of existing alkaline cupric chloride etchants are copper(I) ammonia complex Cu(NH3)4Cl2, ammonium chloride and ammonium hydroxide, wherein Cu(NH3)4Cl2 is obtained from the complexation reaction between ammonium hydroxide and cupric chloride:CuCl2+4NH4OH→Cu(NH3)4Cl2+4H2O
And then, the copper on the printed circuit board is oxidised by [Cu(NH3)4]2+:Cu(NH3)4Cl2+Cu→2Cu(NH3)2Cl.
The copper(I) ammonia complex ions [Cu(NH3)2]+ formed lack etching ability. When excessive NH4+ and Cl− are present in the etchant, [Cu(NH3)2]+ are rapidly oxidised to copper(II) ammonia complex ion [Cu(NH3)4]2+ by oxygen in the air, which can again participate in the copper etching process. Alternatively speaking, copper(II) ammonia complex ions [Cu(NH3)4]2+ are regenerated:4Cu(NH3)2Cl+4NH4Cl+4NH4OH+O2→4Cu(NH3)4Cl2+6H2O
Comparing to existing acidic cupric chloride etchants, alkaline cupric chloride etchants are increasingly used in large-scale industrial production due to their various advantages, including high and easily controlled etching rate, high copper etching ability, easy regeneration of the copper etching agent and low level of etch undercut.
The Etching Process of Existing Alkaline Cupric Chloride Etchants
As the components in the etchant change continuously during the etching process, an automatic detection and charging control machine is generally employed in industrial production to detect a specific density parameter of the etchant, in order to achieve automatic continuous regeneration of the alkaline cupric chloride etchant and thus maintain a stable etching rate. Generally, the etchant can be separated into the following two individual components, and each component can be charged according to its corresponding specific process parameter.
The two individual components are:
1. cupric chloride;
2. sub-etchant: generally a mixture of ammonium hydroxide and aqueous ammonium chloride solution.
In the process of preparing the etchant, the sub-etchant is firstly prepared by dissolving ammonium chloride in water and mixing the resultant aqueous ammonium chloride solution with ammonium hydroxide. The etchant is then obtained by adding cupric chloride into the sub-etchant, until the concentration of copper ions in the solution arrives at a prescribed value.
The mass of added cupric chloride is calculated according to formula 1:
                                                        molar              ⁢                                                          ⁢              mass                        ⁢                                                  ⁢                                                  ⁢                                                  ⁢                          of              ⁢                                                          ⁢                              CuCl                2                                                                                                ⁢                                          molar                ⁢                                                                  ⁢                mass                            ⁢                                                          ⁢                                                          ⁢                              of                ⁢                                                                  ⁢                copper                ⁢                                                                  ⁢                ion                                                    =                                                                                                  mass                    ⁢                                                                                  ⁢                    of                    ⁢                                                                                  ⁢                                          CuCl                      2                                        ⁢                                                                                  ⁢                    to                    ⁢                                                                                  ⁢                    be                    ⁢                                                                                  ⁢                    added                                                                                                                                                                                  ⁢                                          per                      ⁢                                                                                          ⁢                      liter                      ⁢                                                                                          ⁢                      of                      ⁢                                                                                                                        ⁢                                                                                                                      ⁢                      sub                      ⁢                                              -                                            ⁢                      etchant                                                                                                                                                                mass                    ⁢                                                                                  ⁢                    of                    ⁢                                                                                  ⁢                    copper                    ⁢                                                                                  ⁢                    ion                    ⁢                                                                                  ⁢                    to                    ⁢                                                                                  ⁢                    be                                                                                                                    added                    ⁢                                                                                  ⁢                    per                    ⁢                                                                                  ⁢                    liter                    ⁢                                                                                  ⁢                    of                    ⁢                                                                                  ⁢                    sub                    ⁢                                          -                                        ⁢                    etchant                                                                                =                                    mass              ⁢                                                          ⁢              of              ⁢                                                          ⁢              pre              ⁢                              -                            ⁢              added              ⁢                                                          ⁢                              CuCl                2                                                                                                          mass                    ⁢                                                                                  ⁢                    of                    ⁢                                                                                  ⁢                    copper                    ⁢                                                                                  ⁢                    ions                                                                                                                    corresponding                    ⁢                                                                                  ⁢                    to                    ⁢                                                                                  ⁢                                          CuCl                      2                                        ⁢                                                                                  ⁢                    pre                    ⁢                                          -                                        ⁢                    added                                                                                                          (                  Formula          ⁢                                          ⁢          1                )            
wherein the molar mass of cupric chloride is 134.5 g/mol, the molar mass of copper ion is 63.5 g/mol, and the mass of copper ion to be added into per liter of sub-etchant is the prescribed concentration of copper ions in the etchant (unit: g/L).
Assuming that the prescribed mass of copper ion to be added into per liter of sub-etchant is Ag, the mass B of cupric chloride to be added into per liter of sub-etchant, according to formula 1, is (A×134.5÷63.5) g.
A hydrometer on the automatic detection and charging control machine is used for measuring the specific density of the etchant obtained, and the charging control point of a specific density numerical control meter is set according to the measurement of the hydrometer. Alternatively speaking, the control point for charging the sub-etchant is set. Afterwards, the etchant is sprayed onto the surface of the printed circuit board to start the etching operation.
During the etching process, the etchant continuously reacts with copper, and the content of each component in the etchant changes accordingly. In order to achieve a stable etching rate as well as fulfil etching quality requirements, the automatic detection and charging control machine needs to be employed to supplement the sub-etchant. The machine can adjust the specific density of the etchant and keeps it constant at the preset value, so that concentrations of certain components in the etchant remain constant. As the reaction between the etchant and the copper on PCB proceeds, the copper content in the etchant is gradually increased, causing a gradual increase of specific density. When the specific density exceeds the preset value, the automatic detection and charging control machine adds sub-etchant into the etchant by controlling the corresponding charging pump, to reduce the specific density of the etchant. The salt concentration in the etchant is thus kept constant, the ammonium hydroxide and ammonium chloride required for the regeneration of copper(II) ammonia complex is replenished.
Additionally, the pH of the etchant is another important process parameter. A higher concentration of ammonium hydroxide in the etchant leads to larger pH of the etchant, higher regenerating rate of copper(II) ammonia complex, and therefore higher etching rate. In the prior art, the pH of the etchant is controlled by adjusting the pH of the sub-etchant via varying the concentration of ammonium hydroxide in the sub-etchant during its preparation.
An alkaline cupric chloride etchant with pH lower than 8.0 typically contains insufficient ammonium hydroxide, and thus low concentration of ammonium ions. On the one hand, the regeneration rate of copper(I) ammonia complex ion [Cu(NH3)4]2+ is decreased, and the etching rate can hardly reach industrial production requirements. On the other hand, the low concentration of ammonium ions in the etchant results in the presence of a large amount of dissociated chloride ions in the etchant. These chloride ions are in excess, as the slow regeneration reaction is not able to consume all chloride ions, and they are likely to attack the tin etch resist layer instead, leading to corrosion of the tin etch resist layer. On the other hand, the amount of copper ions that cannot be converted into the water-soluble copper ammonia complex ions (due to the low concentration of ammonium ions) is increased, which leads to the formation of a large amount of copper sludge in the etchant. The main components of the copper sludge are CuCl2.NH4Cl.2H2O and cupric hydroxide. When a small amount of copper sludge is generated, it can be advantageously used as banking agent. However, when the amount of the copper sludge is too large, not only is the etching rate seriously affected, but also the pumps and nozzles on etching production lines are easily blocked. Additionally, the surface of the heater may easily form crust, which may lead to increased heat lost, and ultimately damages the heater. Hence at pH<8.0, it is difficult for the existing alkaline cupric chloride etchants to etch with adequate etching rate and etching quality, and so their pH is generally controlled within the range of from 8.3 to 10.
As the pH of the sub-etchant prepared is typically much higher than 8.0, the pH of the etchant is not further controlled and adjusted during the etching process. However, due to the extremely high volatility of ammonia, the concentration of ammonium hydroxide in the etchant will decrease when the ventilation system on the production line is operating on high power or if the etchant is allowed to stand unsealed during a temporary pause of production. The pH of the etchant will decrease, which leads to decreased etching rate. Therefore, if decrease in etching rate is noticed during production or when restarting etching operation after production pause, ammonium hydroxide may need to be replenished. Acid-base titration or a pH meter is required to manually measure the pH of the etchant, and by using known methods in the art, the required amount of ammonium hydroxide needed to be added to adjust the pH of the etchant until it arrives at the original set value is calculated, followed by manual addition of ammonium hydroxide. The existing etchants and etching processes thus have the following disadvantages:                1. During the etching process, the pH of the etchant is only controlled via adjusting the pH of the sub-etchant during its preparation. During the etching operation, there is no real-time and precise pH monitoring of the etchant. As ammonium hydroxide is either evaporated as ammonia, or consumed by the regeneration reaction of copper(II) ammonia complex, the pH of the etchant inevitably decreases during the etching process. When the pH of the sub-etchant prepared is relatively low (e.g. close to 8.0), it will easily drop below 8.0 as etching proceeds. In this case, not only is the etching rate extremely low, but also excessive copper sludge (comprising CuCl2.NH4Cl.2H2O and cupric hydroxide) may be formed, which will damage the etching equipment.        2. When ammonium hydroxide replenishment is necessary (e.g. when the pH of the etchant drops below 8.0), manual calculation of the amount of ammonium hydroxide required needs to be carried out in prior art, which is complicated, time-consuming and error-prone.        3. The replenishment of ammonium hydroxide also involves manual addition of ammonium hydroxide. Ammonium hydroxide is not only irritating and corrosive to eyes, nose and skin, but can also be toxic. Therefore, accidents may easily occur during the manual addition of ammonium hydroxide.        
Etching Outerlayer PCBs with Fine-Line Circuits
As the degree of integration of electronic products increases, the demand for PCBs with fine-line circuits is ever increasing. Generally, “fine-line circuits” refers to circuits with line width and line spacing of below 75 μm, and the etching of such circuits requires the use of higher etching quality etchants. Etching quality is frequently discussed in terms of “etch factor”, which indicates the level of etch undercut. In the etching process, the etchant not only etches downwards, but also etches in the left and right directions, referred to as “etch undercut”. The etch factor (K) is the ratio of etching depth (D) to undercut width at one side (C) (namely K=D/C). When etching depth D is the same, a larger etch factor indicates a smaller undercut width; as the two side walls of the etched circuit is more vertical, the etching quality is better.
A major factor affecting undercut width is the amount of banking agent. Banking agent refers to a substance that is adhered to the two side walls of the circuit and can weaken the degree of attack on the two side walls by the etchant. When the amount of the banking agent is too small, the level of etch undercut is high and the etch factor is small.
In the field of printed circuit board, a fine-line PCB is typically a multiplayer PCB consisting of three or more laminates. A multilayer PCB consists of outerlayers and innerlayers, which are produced separately, before undergoing a laminating process to from the multilayer PCB. The copper foil of the outerlayer is relatively thick, mostly 1 oz or thicker. It is difficult to produce high-quality outerlayer PCBs with fine-line circuits, because:                1. Under the same etching production conditions, a thicker copper foil (i.e. larger etching depth D) leads to a larger undercut width C. Open-circuit and short-circuit is thus more likely to occur. When etching outerlayer PCBs with fine-line circuits (copper foil thickness of 1 oz, line width and line spacing of 50 μm) using existing acidic cupric chloride etchants, the etch factor obtained is typically less than 3, giving non-satisfactory etching results.        2. It has been noticed that alkaline cupric chloride etchants generally give larger etch factors comparing to acidic cupric chloride etchants, and are thus employed in the industrial production of outerlayer PCBs. However, due to the high concentration of ammonium hydroxide in the existing alkaline cupric chloride etchant, the pH of the etchant is relatively high, usually 8.3-10. The large amount of hydroxide ions present in the etchant can attack liquid and dry-film photoresists. It has been found that liquid photoresists are unstable at pH>7.5 and dry-film photoresists are unstable at pH>7.8. Tin etch resists, which are stable in basic conditions, are the only kind of resist layer that can be used. Tin-plated PCBs are relatively expensive, and the circuits developed on tin-plated PCBs are rarely as fine as those developed on PCBs coated with liquid or dry-film photoresists. Producing circuits with line width and line spacing of 75 μm or below using tin-plated PCBs has been found difficult. Theoretically, the corrosion of liquid and dry-film photoresists can be avoided by decreasing the ammonium hydroxide content, and thus lowering the pH of the etchant. As described above, however, the pH of existing alkaline cupric chloride etchants need to be maintained at no less than 8. As a result, the method currently employed in industry for etching fine-line outerlayer PCBs typically involves applying an alkaline cupric chloride etchant on PCBs coated with liquid or dry film photoresists. The pH of the etchant employed is typically very high, in order to achieve a sufficiently high etching rate, so that the etching process is completed before the liquid or dry film photoresists are completely corroded and stripped. However, the fine-line outerlayer PCBs produced using this method have unstable quality, high scrap rate and extremely unsatisfactory results.        
Impacts on the Environment
The existing alkaline cupric chloride etchants give rise to a series of environmental problems                1. Alkaline cupric chloride etchants are typically only used in the etching of tin-plated PCBs. Industrial wastewater produced in the tin plating process can be difficult to treat, and can cause water and soil pollution. Tin smelter workers are continuously exposed to tin oxides in the production of tin-plated PCBs, which can lead to pulmonary stannosis.        2. The current method of treating alkaline cupric chloride etchant waste generally involves copper recovering and the subsequent sub-etchant recycling. The said etchant waste is mostly etchant overflowed from etchant tank during the etching process. Organic solvents are typically used to extract and recover copper from the etchant waste, albeit with high recovering cost and may contaminate the recycled sub-etchant. The organic solution waste produced can further cause environmental pollution. Copper recovering can alternatively be achieved using acid-base neutralisation, which involves adding a certain amount of hydrochloric acid into the alkaline cupric chloride waste, until its pH is adjusted to 6-7. At this point, CuCl2.NH4Cl.2H2O and cupric hydroxide precipitates are formed, which can react with sulphuric acid to form copper sulphate. The copper sulphate formed can be converted into refined copper by electrolysis. However, since the concentration of ammonium hydroxide in current alkaline cupric chloride etchants is relatively high, a large amount of HCl is required for neutralisation. As a result, a large amount of ammonium chloride solution is produced, which is difficult to be completely recycled and treated, and is mostly discharged as waste, causing huge waste of resources. Therefore, acid-base neutralisation is seldom applied at present.        3. As a high concentration of unreacted ammonium hydroxide is present in existing alkaline cupric chloride etchants, a large amount of irritating ammonia gas may be evolved by evaporation. Symptoms of mild ammonia gas poisoning by inhalation include rhinitis, laryngitis, sore throat and hoarse voice, and symptoms of moderate to severe ammonia gas poisoning include shortness of breath and asphyxia resulting from collapse of airways. This can be hazardous to the production staff, and can also cause air pollution.        
In summary, existing alkaline cupric chloride etchants can corrode liquid and dry-film photoresists, causing various difficulties in the production of high-quality PCBs with fine-line circuits. Furthermore, a series of environmental problems may arise in the production of such PCBs.